Hi, I have a Chris craft 300 with 2 Volvo penta 5.0gxi with 200 hs. One of them, after 2 mins at 3500 rpm, started lowering down the rpm's, backfiring and up and down until throttle was lowered down. After $800 and a new fuel pump assembly, we go out for a test. 4 mins runnig at 3500 rpm, and...